摘要
The recent discovery of superconductivity in the bilayer Ruddlesden-Popper nickelate
La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>
under high pressure has generated much interest in the superconducting pairing mechanism of nickelates. Despite extensive work, the superconducting pairing symmetry in
La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>
remains unresolved, with conflicting results even for identical methods. We argue that different superconducting states in
La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>
are in close competition and highly sensitive to the choice of interaction parameters as well as pressure-induced changes in the electronic structure. Our study uses a multiorbital Hubbard model, incorporating all Ni
3
d
and O<sub>2</sub>
p
states. We analyze the superconducting pairing mechanism of
La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>
within the random phase approximation and find a transition between
d
-wave and sign-changing
s
-wave pairing states as a function of pressure and interaction parameters, which is driven by spin fluctuations with different wave vectors. These spin fluctuations with incommensurate wave vectors cooperatively stabilize a superconducting order parameter with
d
x
2
−
y
2
symmetry for realistic model parameters. Simultaneously, their competition may be responsible for the absence of magnetic order in
La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub>
, demonstrating that magnetic frustration and superconducting pairing can arise from the same set of incommensurate spin fluctuations.</p>
材料
方法
- Random phase approximation
- Multiorbital Hubbard model
关键词
- incommensurate spin fluctuations
- competing pairing symmetries
- d wave pairing
- s wave pairing
亮点
- Magnetic frustration and superconducting pairing can arise from the same set of incommensurate spin fluctuations.
- Competing fluctuations may explain the absence of magnetic order in La3Ni2O7.
结论
- A transition between d-wave and sign-changing s-wave pairing states occurs as a function of pressure and interaction parameters.
- Spin fluctuations with incommensurate wave vectors stabilize dx2-y2 symmetry for realistic parameters.
主要论断
- Incommensurate spin fluctuations with multiple wave vectors exist in La3Ni2O7 and are crucial for superconducting pairing.
- 证据: Calculated spin susceptibility shows peaks at q1~(π/2,π/2) and q2~(7π/10,7π/10) (Fig.2),Strength of incommensurate peak controlled by Hund's rule coupling J
- Superconducting pairing symmetry in La3Ni2O7 can be either s±-wave or dx2-y2-wave depending on pressure and interaction parameters.
- 证据: Phase diagram (Fig.3) shows transition between s± and d-wave as function of U, J, and pressure,At P=24.6 GPa and U=3 eV, J=0.75 eV, d-wave is dominant
- The absence of magnetic order in La3Ni2O7 is due to magnetic frustration arising from competing spin fluctuations at different wave vectors.
- 证据: Same spin fluctuations that cooperate for pairing compete for magnetic ordering (discussion),RPA instability regions indicate potential spin-density wave but are suppressed at high pressure
研究流程
- DFT band structure calculation — The electronic structure shows strong p-d hybridization and a shallow hole pocket with Ni 3dz2 character near Fermi level.
- 材料: La3Ni2O7 crystal structure under pressure (space group I4/mmm)
- 方法: Full potential local orbital (FPLO) basis; Generalized gradient approximation (GGA)
- 观察: Electronic band structure and density of states (Fig.1); Identification of Ni 3d and O 2p orbital contributions
- Tight-binding model construction — A realistic multiorbital model is essential to capture correct orbital weights and Fermi surface nesting.
- 材料: DFT band energies and Fermi surface
- 方法: Projective Wannier functions
- 观察: Model reproduces DFT band structure and Fermi surface accurately
- Spin susceptibility calculation — Incommensurate spin fluctuations at q2 are controlled by Hund's rule coupling J and lead to magnetic frustration.
- 材料: Tight-binding Hamiltonian; Hubbard-Kanamori interaction parameters U, J
- 方法: Random phase approximation (RPA)
- 观察: Spin susceptibility shows peaks at q1=(π/2,π/2) and incommensurate q2~(7π/10,7π/10); Type I and Type II nesting vectors identified
- Superconducting pairing analysis — Superconducting pairing symmetry is sensitive to pressure and interaction parameters; dx2-y2-wave pairing is stabilized by cooperation of incommensurate spin fluctuations.
- 材料: Spin susceptibility; Linearized Eliashberg equation
- 方法: RPA pairing vertex; Eigenvalue analysis
- 观察: Phase diagram showing transition between s±-wave and dx2-y2-wave pairing; d-wave pairing stabilized for realistic parameters with J and U
- Interpretation and conclusions — The model provides a unifying framework to understand contradictory results from few-orbital models; predicts tuning between s± and d-wave by pressure or doping.
- 材料: Results from susceptibility and pairing calculations
- 方法: Comparison with literature
- 观察: Spin fluctuations with two wave vectors cooperate for d-wave pairing but compete for magnetic ordering; Pressure tunes crystal field splitting and bandwidth